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J Comp Physiol A DOI 10.1007/s00359-007-0209-y O RI G I NAL PAPE R Listening when there is no sexual signalling? Maintenance of hearing in the asexual bushcricket Poecilimon intermedius Gerlind U. C. Lehmann · Johannes Strauß · Reinhard Lakes-Harlan Received: 25 October 2006 / Revised: 12 December 2006 / Accepted: 7 January 2007 © Springer-Verlag 2007 Abstract Unisexual reproduction is a widespread phenomenon in invertebrates and lower vertebrates. If a former sexual reproducing species becomes parthenogenetic, we expect traits that were subject to sexual selection to diminish. The bushcricket Poecilimon intermedius is one of the few insect species with obligate but diploid parthenogenetic reproduction. We contrasted characters that are involved in mating in a sexually sibling species with the identical structures in the parthenogenetic P. intermedius. Central for sexual communication are male songs, while receptive females approach the males phonotactically. Compared to its sister-species P. ampliatus, the morphology of the hearing organs (acoustic spiracle, crista acustica) and the function of hearing (acoustic threshold) are reduced in P. intermedius. Nonetheless, hearing is clearly maintained in the parthenogenetic females. Natural selection by acoustic hunting bats, pleiotropy or a developmental trap may explain the well maintained hearing function. Keywords Neuroanatomy · Bioacoustics · Auditory ecology · Regression · Vestigialization G. U. C. Lehmann (&) Institut für Zoologie, AG Evolutionsbiologie, Freie Universität Berlin, Abteilung Evolutionsbiologie, Königin-Luise-Straße 1-3, 14195 Berlin, Germany e-mail: [email protected] J. Strauß · R. Lakes-Harlan Institut für Tierphysiologie, AG Integrative Sinnesphysiologie, Universität Gießen, Gießen, Germany Introduction Traits under relaxed selection are expected to become reduced or disappear completely, a process called vestigialization. Vestigialization is a widespread phenomenon that highlights the importance of natural selection for the maintenance of adaptive traits (Fong et al. 1995; Porter and Crandall 2003). Vestigialization could proceed more quickly if trait reduction was associated with increased Wtness. Reduction of the vestigial trait may be selected directly if the trait becomes a Wtness liability, like some sexually selected traits (Wiens 2001). Loss of traits has been reported in a wide variety of organisms, such as cave-dwelling animals (Wilkens et al. 2000), subterranean mammals (Nevo 1999) or island birds in the absence of predators (McNab 1994). In parthenogenetic species or populations, traits historically involved in sexual reproduction are no longer under selection and potentially subject to vestigialization (Carson et al. 1982). Asexuality may directly aVect the mutation rate due to the loss of the sexual phase and lead to the cessation of sexual selection (Normark et al. 2003). The evolution of signalling is at the heart of many evolutionary discussions (GreenWeld 2002). Hearing in Orthoptera is associated with two functions: intraspeciWc communication (e.g. mate recognition and localization) and predator recognition and avoidance (Bailey 1991; Gwynne 2001; Gerhardt and Huber 2002; Robinson and Hall 2002). Both functions might be driven by diVerent requirements on auditory performance and can be caused by distinct processes—sexual selection (in the case of intraspeciWc communication) and more general natural selection. It is argued that in a variety of cases the actual structure and performance of the 13 J Comp Physiol A acoustic system reXects merely the balance between sexual selection due to elaborated signalling and a counteracting natural selection (Zuk and Kolluru 1998; GreenWeld 2002). Thus, their respective inXuence on a given hearing system is hard to distinguish causally. The tympanal auditory system of tettigoniid grasshoppers is a highly ordered sensory system (Römer 1983; Lakes and Schikorski 1990). The receptors are located in a linear array of chordotonal cells, called the Crista acustica. The airborne sound is transferred to the receptors by a highly specialized auditory spiracle or trachea (Nocke 1975; HoVmann and Jatho 1995). Tibial tympana contribute to the sensitivity of the system (Bangert et al. 1998). Evidence exists that intraspeciWc communication in tettigoniids evolved before predation by ultrasound emitting bats (Stumpner and von Helversen 2001), but the inXuence of predation on audition is not easily quantiWed (Hoy 1992). Since the Miocene bats have become a signiWcant predation risk and can be considered as a major force in shaping acoustic behaviour in tettigoniids (Hoy 1992; Yager 1999; Lehmann 2003; Fullard et al. 2004). The ensiferan genus Poecilimon, including more than 120 species, has served as model for evolutionary studies (Heller 1997). The bushcricket P. intermedius (Fieber, 1853) is a good system to investigate the inXuence of sexual and natural selection on the hearing system. In P. intermedius only females are known over its distribution range (Heller and Lehmann 2004) and these females are diploids (Warchalowska-Sliwa et al. 1996). In rearing experiments, this species was capable of all female reproduction and could not be treated to revert to sexual reproduction (Lehmann and Bourtzis, submitted). Parthenogenesis in P. intermedius can be classiWed according to Suomalainen et al. (1987) as constant obligate thelytoky (unfertilised eggs develop into females) and somatic diploidy. The ecological pattern is geographical parthenogenesis (Peck et al. 1998; Kearney 2005), with a more northerly and larger distribution area than sexual Poecilimon species (Heller and Lehmann 2004). As P. intermedius is an obligate parthenogenetic, the intraspeciWc role of acoustic communication in mate recognition and Wnding is lost and therefore sexual selection pressure on auditory performance can be assumed to no longer exist. Reduction could act on several levels of the hearing system, from the sensory structures to the neuronal network in the central nervous system (CNS). The objectives of the present study were the analysis of peripheral sensory structures, which provide input for higher integration centres. Comparing the parthenogenetic species P. intermedius 13 with its comparatively recently separated closest relative P. ampliatus, identiWed by DNA sequencing (Lehmann et al., submitted), allows the comparison of auditory system structure and function in presence and absence of intraspeciWc communication. For the Wrst time two sister-species with alternative reproductive modes were tested for the design of the hearing structures as the acoustic spiracle, the entrance of sound into the trachea, and the tympana as well as the neuroanatomy of the crista acustica. Electrophysiological recordings of auditory thresholds determined physiological performance. We expect the auditory performance of P. intermedius to be reduced in sensitivity compared to its sister-species P. ampliatus. Materials and methods The members of the P. ampliatus-group are medium size, Xightless bushcrickets (Heller and Lehmann 2004). Adult P. intermedius (WEBER, 1853) and P. ampliatus BRUNNER VON WATTENWYL, 1878 were individually housed in 200 ml boxes (Drosophila rearing boxes, Greiner Bio-one GmbH, Frickenhausen, Germany; http://www.GreinerBioone.com) and provided with a mixture of Taraxacum leaves and Xowers and mixed species pollen available from a health food store ad libitum. Morphology The size of the acoustic spiracle was determined using scanning electron microscopy. Females of both species were decapitated and the ventral lobe of the pronotum was removed to expose the spiracle. The isolated thoraces with the visible spiracle were transferred to small caps covered with 10-m gauze and dehydrated in increasing concentrations of ethanol from 70 to 100%. Critical point drying was performed with CO2 (Balzers CPD 030, Balzers AG, Lichtenstein). The specimen were Wxed on stubs with double-sided adhesive tape and coated with gold (Balzers SPD 040). Pictures were obtained by a Philips SEM 515. Digitalized pictures of the spiracles were measured, using the program Scion Image Beta 4.03 for Windows (http://www.scioncorp.com). As references for body size we measured the length of the hind femur and the pronotum for all females with a Mitutoyo calliper rule (accuracy 0.01 mm). The size the tympana were measured with help of a microscope (Leitz), using an ocular micrometer (accuracy 0.025 mm). The maximum extensions in the proximo-distal and the dorso-ventral axis were measured J Comp Physiol A for both anterior and posterior tympana, respectively. Some tympana were processed for scanning electron microscopy as described above. loscope and transferred to earphones. The auditory threshold was determined acoustically and visually as the Wrst signal intensity, which excited a neuronal response. Neuroanatomy Results Hearing threshold For electrophysiological recordings, animals were Wxed on their back on a metal holder (seven females of each species). The forelegs were attached with wax to metal wires, resembling the normal leg position. The recording took place in a Faraday cage that was covered with sound dampening material. Stimulation Acoustic stimuli were delivered from a broad-band loudspeaker (Dynaudio, DF 21/2) located at a distance of 38 cm from the animal. The stimuli were computer generated and ampliWed (Lang et al. 1993). Frequencies between 3 and 40 kHz were presented to the animals. Each frequency–intensity combination was tested 5 times with a stimulus duration of 100 ms each. The intensity increased from 30 to 50 dB SPL in steps of 5 dB and from 50 to 80 dB in steps of 10 dB SPL. Calibration measurements were carried out with a sound level meter (Brüel and Kjær 2203) and a microphone (Brüel and Kjær 4135). Recordings Summed action potentials from the tympanal nerve were recorded near the entrance of the nerve into the prothoracic ganglion. Recordings were carried out with a silver wire hook electrode. The reference electrode was inserted between the connectives close to the recording site. The signal was ampliWed 1,000 fold by a preampliWer (Tektronix T122) and displayed on an oscil- Spiracle size The general size of P. intermedius females was smaller than in P. ampliatus females, measured as pronotum (4.65 § 0.17 mm vs. 5.33 § 0.25, T test: T = 8.73, df = 28, P < 0.001) and hind femur length (14.98 § 0.48 mm vs. 16.20 § 0.44, T test: T = 7.23, df = 28, P < 0.001). Despite this size diVerence between the species, the correlation between hind femur and spiracle size is only weak (Fig. 1, correlation coeYcient for both species combined: R2 = 0.18) and even less when analysed separately for either P. intermedius (R2 = 0.06) or P. ampliatus (R2 = 0.09). The general auditory anatomy of females of both P. intermedius and P. ampliatus is typical of tettigoniid bushcrickets. The tracheal system of the prothoracic leg is expanded in the tibia to form air-Wlled vesicles, which lie beneath the two tympanal membranes. The leg trachea opens to the exterior through an acoustic spiracle. This acoustic spiracle on the lateral side of the prothorax functions as input for the hearing system (Fig. 2). The acoustic spiracle has a comparatively small opening in P. intermedius of mean (§SD) 0.94 § 0.17 mm2 (n = 17). As such the spiracle is signiWcantly smaller than in the sister-species P. ampliatus (1.11 § 0.16 mm2, n = 13) (T test: T = 2.70, df = 28, P < 0.05). Females of P. intermedius have a 15% smaller spiracle size in comparison to P. ampliatus 1,6 1,4 1,2 Spiracle [mm2] For marking the sensory cells of the complex tibial organ, tympanal nerves were backWlled with a CoCl2 solution (Lakes-Harlan et al. 1991). Forelegs were cut oV and attached with needles to a silicone covered petri dish bottom. The tympanal nerve was dissected from the femur and placed into a glass capillary Wlled with 5% CoCl2 and incubated at 4°C for 2 days. After incubation, the tarsus was cut oV and the tibia was opened dorsally. The CoCl2 was precipitated with ammonium sulphide (1% solution for 10 min), and the preparations were Wxed in 4% paraformaldehyde. The neuronal marking was enhanced by the silver intensiWcation method (Bacon and Altman 1977). All preparations were dehydrated (in 30, 50, 70, 90, 96, 100% Ethanol for at least 60 min each) and cleared in methyl salicylate. The complex tibial organ was documented by pictures and drawings using a Leitz Dialux microscope and a drawing tube (n = 3 for P. ampliatus and n = 5 for P. intermedius). 1,0 0,8 0,6 ampliatus intermedius 0,4 0,2 12 14 16 18 20 Hind Femur [mm] Fig. 1 Spiracle size compared to length of the hind femur in females of P. intermedius (n = 17) and P. ampliatus (n = 13) 13 J Comp Physiol A membranes on the anterior and posterior side beneath the knee (Fig. 4). The size of the tympana was identical between the two species (see Table 1) for the proximodistal axis (two-way ANOVA: species: F1,32 = 1.81, P = 0.18 NS) and the dorso-ventral axis (F1,32 = 0.00, P = 0.96 NS) for both, the anterior and the posterior membrane. However, the anterior tympana were signiWcantly larger than the posterior membranes in both species in the proximo-distal axis (F1,32 = 42.12, P < 0.001), as well as in the dorso-ventral axis (F1,32 = 14.08, P < 0.001). The interaction term was not signiWcant for both measurement axes, which indicates that there was no inXuence of the species on the size reduction of the posterior tympana compared to the anterior side (proximo-distal axis: F1,32 = 0.84, P = 0.36 NS, dorso-ventral axis: F1,32 = 2.97, P = 0.09 NS). Neuroanatomy of the Crista Acustica Fig. 2 SEM picture of the acoustic spiracle of a female of P. intermedius (a) and P. ampliatus (b). Scale bar 0.1 mm The general organization of the three receptor organs within the proximal tibia of both P. ampliatus and P. intermedius is the same as in most other tettigoniids. The complex tibial organ is divided into three sensory organs: the subgenual organ, the intermediate organ and the crista acustica (Fig. 5). The subgenual organ forms a “sail”-like structure extending from anterior into the haemolymph channel of the leg. In both species, the females (Fig. 3). Nonetheless, the variance within individuals was similar for both species (Bartlett test for equal variances: 2 = 0.14, df = 1, P = 0.71 NS). Tympanum size The most obvious structures of the auditory system in the forelegs of bushcrickets are two tympanic P < 0,05 1,4 Spiracle [mm2] 1,3 1,2 1,1 1,0 0,9 0,8 0,7 0,6 ampliatus intermedius Fig. 3 Area of the acoustic spiracle of the P. ampliatus (n = 13) and P. intermedius (n = 17), given as means (§SD) 13 Fig. 4 Tympanum of P. intermedius female anterior. The membrane is located just under the knee, which is to the top. Scale bar 0.1 mm J Comp Physiol A Table 1 Tympana sizes given as means [§SD] in mm for the two species n P. intermedius P. ampliatus 11 23 Anterior tympanum Posterior tympanum Proximo-distal Dorso-ventral Proximo-distal Dorso-ventral 0.75 § 0.02 0.75 § 0.05 0.31 § 0.02 0.30 § 0.02 0.63 § 0.04 0.67 § 0.09 0.28 § 0.02 0.29 § 0.02 Fig. 5 Drawings of the array of the receptor cells of the complex tibial organ in the foreleg of a female of P. ampliatus (a) and P. intermedius (b). The three parts of the organ are marked for P. ampliatus (CA crista acustica, IO intermediary organ, SGO subgenual organ). A sensory neuron (SN), with its dendrite (D) and a cap cell (CC) of a scolopidium is indicated exemplarily in the crista acustica of P. intermedius anterior scolopidial receptor cells stretch along the sail and probably react to vibrational stimuli. The intermediate organ is similar in both species and contains about 10–11 scolopidial receptor cells. The crista acustica (Fig. 6) is undoubtedly the most interesting part of the sense organ array as its primary function in tettigoniids is to transduce airborne sound. The receptor cells form a linear row along the length axis of the leg. Generally, the dendrites of the sensory cells as well as their cell bodies become smaller towards the distal end of the crista acustica. The decrease of size is even more marked in the cap cells to which the sensory neurons are attached (Fig. 6). Usually in both species about six to eight cap cells at the distal end are much smaller than the proximal cells. Females of P. intermedius have a lower number of receptor cells in the crista acustica (16–18 cells, n = 5) in comparison to those of P. ampliatus (21 cells, n = 3) (Fig. 5). The overall length of the crista acustica is in mean (§SD) 0.45 § 0.04 mm in P. ampliatus (n = 3) and 0.35 § 0.03 mm in P. intermedius (n = 2). Even with such a small sample size this diVerence is signiWcant (T test: T = 3.43, df = 3, P < 0.05). Fig. 6 Lateral view of complex tibial organ and the crista acustica in a foreleg of P. intermedius (left) and P. ampliatus (right), labeled with cobalt chloride. Scale bar 0.1 mm Hearing threshold Extra cellular recording the tympanal nerve show that both species perceive auditory stimuli. P. ampliatus has a low acoustic threshold of, meaning that the nerve is most sensitive, between 7 and 10 kHz (Fig. 7). The energy of the male song is mainly in the range between 20 and 40 kHz (Heller 1988). In this range the threshold for nervous response is about 55–60 dB SPL, indicating a mismatch in tuning to the male song spectrum. P. intermedius females have a hearing threshold with some small diVerences to that of P. ampliatus (Fig. 7). Auditory tuning is broad, and the threshold curve is rather Xat, lacking a clear sensitivity peak. Most sensitive hearing is around 10 kHz and around 65 dB SPL at high frequencies used in sexual species to attract mates. On average, the hearing threshold in P. intermedius females was 3–7 dB higher than in P. ampliatus females at all frequencies, with signiWcant diVerences in all frequency ranges. This diVerence corresponds well with the reduced spiracle size and reduced number of sensory cells in P. intermedius. However, both P. ampliatus and the parthenogenetic species 13 J Comp Physiol A 85 P. ampliatus Threshold [dBSPL] 80 P. intermedius 75 70 65 60 55 50 45 40 35 0 10 20 30 40 50 Frequency [kHz] Fig. 7 Mean auditory threshold curves (§SD) for P. ampliatus (n = 8) and P. intermedius (n = 8) P. intermedius showed reduced sensitivity, especially at higher frequencies, compared to bushcrickets with larger spiracles (Stumpner and Heller 1992). Discussion As P. intermedius is obligatory parthenogenetic, the intraspeciWc role of acoustic communication in mate recognition and Wnding (Gwynne 2001) is lost, and therefore sexual selection pressure on auditory performance no longer exists. Natural selection due to predator recognition may still be acting on the tympanal organ. As expected, females of the parthenogenetic P. intermedius show reduction in the hearing structures compared to females of its sexual sister-species P. ampliatus. However, the anatomical and functional vestigialization is rather small, given the complete loss of intraspeciWc acoustic communication. The split of the two species is of comparatively young age, given the low genetic diVerences (Lehmann et al., MS). The inferred age of separation of the split between the two species might be assumed up to 200,000 years, leaving enough time for a signiWcant respond to the altered reproductive mode. However, the origin of parthenogenesis in P. intermedius might be younger than the split. In both species the complex tibial organ characteristic for Tettigoniid bushcrickets (Lakes and Schikorski 1990) is fully developed. Tympana are present and do not show size diVerences between the sister-species. The total size of the tympana in both these species and other Poecilimon species (Stumpner and Heller 1992) is small compared to other bushcrickets (Bangert et al. 1998). In both P. intermedius and P. ampliatus the crista acustica has a relatively small number of receptor cells compared to Tettigoniids in general, which often possess 20–40 cells (Lakes and Schikorski 1990; Robin- 13 son and Hall 2002). The oak bushcricket Meconema thalassinum (Schumacher, 1973, 1979), which has changed intraspeciWc communication towards leaf drumming (Sismondo 1980), and the Australian Phasmodes ranatriformes (Lakes-Harlan et al. 1991), which is an atympanate species, have both receptor cell numbers as low as the asexual P. intermedius. P. intermedius has on average four receptor cells less than its sister-species P. ampliatus. Such a diVerence is rather large, given that bushcricket species in a genus diVer in mean by two (Ephippiger: Schumacher 1973, 1979) or just one (Tettigonia: Kalmring et al. 1995) receptor cell in the crista acustica. This reduction in cell number is a clear evidence of vestigialization (Fong et al. 1995). Variation of a vestigialized trait might be greater as an eVect of a gradual reduction process, leaving traces of the reduction steps behind (West-Eberhardt 2003). In contrast parthenogenetic reproduction is said to be connected with lower variation, due to the lack of recombination (Felsenstein 1974). The intraspeciWc variation of the number of crista receptors is usually restricted to 1 or 2 cells (Lakes and Schikorski 1990; Robinson and Hall 2002), to which the data from both P. ampliatus and P. intermedius accord. Therefore the degree of variation seems unaVected by the strong vestigialization process of the cell number. The acoustic spiracle is the main source of sound entry in bushcrickets (Robinson and Hall 2002). These spiracles are smaller in the two species we studied than in any known bushcrickets of similar size (Bangert et al. 1998). Only the Australian Kawanaphila species have similar small openings (Bailey and Römer 1991; Mason and Bailey 1998), but they are much smaller, weighing only 10–20% of our species (Simmons and Bailey 1990; Lehmann and Lehmann 2007). The diVerences between P. ampliatus and P. intermedius in spiracle size are, even if signiWcant, rather small. Nonetheless, this small diVerence correlates with a reduced auditory performance of P. intermedius. Both Poecilimon species hear airborne sound over the entire spectrum tested, showing relatively high hearing thresholds, meaning less sensitivity, compared to other Tettigoniids, which can achieve thresholds at intensities of 25–30 dB SPL (Gerhardt and Huber 2002). This overall low sensitivity Wts to the relatively small spiracle, as hearing threshold and spiracle size were closely linked in three other Poecilimon species (Stumpner and Heller 1992). Evolutionary implications of reduction Natural selection tends to act most strongly on aspects of the phenotype at relatively high levels of biological J Comp Physiol A organization because they are the most strongly correlated with Wtness (Ketterson and Nolan 1999; Irschick and Garland 2001; Kingsolver and Huey 2003; Costa and Sinervo 2004). Complex traits with many subordinate traits at lower levels of biological organization may be aVected by natural selection (Swallow and Garland 2005). Thus, the evolutionary response to selection on such complex traits necessarily entails associated changes in aspects of morphology, physiology and biochemical pathways (Ghalambor et al. 2003; Sinervo and Calsbeck 2003). Reduction in the functionality of a trait can aVect aVerent sensory structures, central neural structures, and eVectors like muscles or glands (Katz and HarrisWarwick 1999). For tympanal organs, reduction has occurred in several insect taxa (Stumpner and von Helversen 2001). For those investigated in detail with respect to neuroanatomy and physiology, diVerent levels of reduction have been documented, which ultimately aVect the species speciWc behaviour. One is the loss of behavioural responses to auditory stimuli that have lost their biological signiWcance, as observed in a moth (Fullard et al. 2004). Females of P. intermedius did not show phonotaxis when presented with the P. ampliatus male song (Lehmann MS). In species of the P. propinquus-group, female song preferences were not species-speciWc, but correspond well with the similarity of song patterns produced by conspeciWc and heterospeciWc males (A.W. Lehmann, personal communication, 2006). Additionally, peripheral structures can be reduced even to the obliteration of tympana and reduction of auditory trachea as in the mimetic tettigoniid Phasmodes (Lakes-Harlan et al. 1991). Comparative studies have led to the conclusion that such structures can be readily modiWed by evolution, while neuronal sensory structures are more conservative and cannot (Dumont and Robertson 1986; Yager 1999). Even in a secondarily atympanate species of Tettigoniidae the crista acustica is present (Lakes-Harlan et al. 1991), which indicates evolutionary conservatism in this structure. The basic organization of the nervous systems tends to be highly conservative (Katz and Harris-Warwick 1999). In invertebrates, where individual neurons can be unambiguously identiWed from animal to animal within a species, homologous neurons can be identiWed in disparate groups of insects (Edwards 1997). These neurons can exhibit certain morphological modiWcations that are taxon speciWc (Buschbeck and Strausfeld 1996). The most common diVerence seen in closely related species is a change in the number of cells of a particular type. The organizational features of some neuronal structures seems to provide a simple mecha- nism for addition or subtraction of identical units, leading to computationally important changes in neuronal number (review in Katz and Harris-Warwick 1999). Plasticity in P. intermedius follows a pattern of vestigialization of the accessory spiracle and receptor number that maintains hearing but aVects sensitivity. The reduced hearing threshold does not imply changed central organization but follows from a reduced input by the smaller spiracle. It will be interesting to evaluate possible changes in the central neuronal network of auditory processing. The basic auditory networks are known in tettigoniids (Stumpner 1996) and changes might be followed to a single cell level. In other insects like moths (Fullard et al. 2004) behavioural changes in a predator free islands have been documented, with the loss of a defensive behaviour against bat ultrasound emissions. While the single auditory receptor neuron still exists and possesses similar sensitivity thresholds compared to the cell of recently introduced species, it exhibits reduced Wring activity (Fullard et al. 2007). Similar to our bushcricket species, in this moth system the neural regression is incomplete and the sensoribehavioural integration decays gradually following the removal of the selection pressure exerted by echolocating bats. Two evolutionary processes can underlie the reduction in sensory structures such as tympanal organs. These are adaptive and random changes (Fong et al. 1995; Porter and Crandall 2003). Adaptation would reduce sensory organs/structures in order to save energy and material by not building and maintaining them. Random mutations could equally lead to a reduced organ once selection pressure is weakened after a reduced biological function. For the Poecilimon species reported here, the causation of reduction is not easily identiWed. P. intermedius has four crista receptors less than P. ampliatus, which is a 19% reduction, but still covers the entire frequency range tested. The reduced sensitivity correlates with the spiracle size. As the crista may still function in predation avoidance, maintenance of the hearing organ would still be caused by natural selection. Ultrasound emitting bats are regarded as a strong natural selection force in maintenance of hearing function in insects (Hoy 1992; Yager 1999). In bushcrickets acoustic perception serves both anti-predatory and sexual functions. If anti-predator functions are relatively strong, the evolutionary loss of sexual signalling by parthenogenesis may not be accompanied by a major reduction in hearing as found in P. intermedius. How strong this selection pressure actually is has to be demonstrated experimentally, but following our results we assume natural selection to be a stronger force than sexual selection. 13 J Comp Physiol A Acknowledgments We thank K.-G. 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